The present invention is directed to a device for optically measuring distance.
Optical distance-measuring devices as such have been known for a long time, and they are now sold commercially in large quantities. These devices emit a modulated light beam that is directed toward the surface of a desired target object whose distance from the device is to be determined. A portion of the returning light that has been reflected or scattered by the target object is detected by the device, and it is used to determine the distance in question.
The application range of distance-measuring devices of this type generally ranges from a few centimeters to several hundred meters.
Depending on the paths to be measured and the reflectance of the target object, different requirements result for the light source, the quality of the measurement beam, and the detector.
The optical distance-measuring devices known from the related art basically belong to two categories, depending on the configuration of the transmission and reception channels present in the device.
In one category, there are devices with which the transmission channel is located a certain distance away from the reception channel, so that the optical axes extend in parallel with each other but at a distance away from each other. The other category includes monoaxial measuring devices with which the reception channel extends coaxially with the transmission channel.
The former, biaxial measurement systems have the advantage that a complex beam-splitting system is not required to select the returning measurement signal, thereby also enabling, e.g., optical crosstalk from the transmission path directly into the reception path to be suppressed to a greater extent.
Biaxial distance-measuring devices have the disadvantage, however, that detection problems may arise when close-range distance measurements are performed, due to a parallax. In this case, the image of the target object on the detector surface—the image being located unambiguously on the detector even when target distances are great—moves increasingly further away from the optical axis of the reception path as the measurement distance decreases, and the beam cross-section in the detector plane changes markedly.
As a result, the measurement signal that is detected may approach zero in the close range of detection, i.e., when the distance between the target object and the measuring device is short, if no further measures are taken in the device.
Although measuring devices of this type may be optimized for a certain distance range, this requires that the measuring range that is actually accessible to the measuring device be limited substantially.
Publication DE 10 130 763 A1 makes known a device for optically measuring distance over a large measuring range that includes a transmission unit with a light source for emitting modulated, optical radiation toward a target object, and with which the receiving unit that includes an optical detector located in this measuring device—which serves to receive the optical radiation returning from the target object—is located on a reception axis, which is located at a distance away from the optical axis. The active, photosensitive surface of the detector of the reception unit described in DE 10 130 763 A1 tapers in the direction of a beam displacement for decreasing target object distances that results due to a parallax of the returning measurement radiation.
Publication DE 10 051 302 A1 makes known a laser distance-measuring device for close range and long-range that includes a special receiver with a transmission channel and a reception channel. The transmission channel is composed of a transmission lens, in whose focal point a laser light source is located. The reception channel is composed of a reception lens, in whose focal plane a receiver system is located. The optical axes of the transmission lens and the reception lens extend in parallel with each other for a finite distance. The receiver system of the laser distance-measuring device described in DE 100 51 302 A1 is a photodiode chip system with at least two active photodiode surfaces located on a straight line that intersects the optical axes of the transmission and reception lens of this device.
The object of the present invention is to ensure—based on a device for optically measuring distance according to the related art—that the most constant reception signal possible may be measured across the largest possible measuring range.